Development of lithium-ion secondary batteries with high energy density serving as power sources of battery electric vehicles and other types of vehicles has been promoted energetically in recent years. In secondary batteries for vehicle installation, higher electric current flows through the battery in comparison with secondary batteries for cellular phones or other devices. Therefore, prevention of the mixing of water into the battery and the leakage of the electrolyte is highly essential and the sealability of the battery is a critical issue. Patent Document 1, for example, discloses a prismatic secondary battery in which a winding electrode is stored in a battery case, the electrolyte is injected into the battery case, and thereafter the battery housing is hermetically sealed up with a cover welded to the battery case. That in Patent Document 2 has a structure in which a convex part is formed on the back side of the cover.